Ultrasound Physics Module One Lecture PDF

Introduction to Ultrasound Physics MODULE ONE As we go through this course, look for the star to indicate information that is highly likely to be on the SPI board exam “ SOUND ”  TYPES OF WAVES Types of waves: Heat, sound, magnetic, light… All carry energy from point A to...

Introduction to Ultrasound Physics MODULE ONE As we go through this course, look for the star to indicate information that is highly likely to be on the SPI board exam “ SOUND ”  TYPES OF WAVES Types of waves: Heat, sound, magnetic, light… All carry energy from point A to point B  SOUND WAVES Sound waves are… MECHANICAL WAVES Require a medium to transfer their energy through  SOUND WAVES Thus… Sound waves cannot travel in a vacuum SOUND WAVES Effects of Effects of Medium on Wave on Wave Medium Acoustic Biologic Effects Propagation Properties ACOUSTIC VARIABLES ACOUSTIC VARIABLES Sound waves are created by oscillations in acoustic variables ACOUSTIC VARIABLES Three acoustic variables help to distinguish sound waves between other waves: PRESSURE DENSITY DISTANCE Concentration Concentration Measure of of force in an of mass in a particle in area volume motion Pascals (Pa) kg/cm³ cm, mm TRANSVERSE VS. LONGITUDINAL WAVES  TRANSVERSE vs. LONGITUDINAL WAVES TRANSVERSE LONGITUDINAL Particles move in a direction that is Particles move in a direction that is perpendicular to the direction the wave parallel to the direction the wave propagates from. propagates from. Sound w av e sa longit re u al wa din ve s IN-PHASE AND OUT-OF-PHASE WAVES AND INTERFERENCE  IN-PHASE vs. OUT-OF-PHASE In-phase waves: When wave peaks and troughs of the waves occur at the same location IN-PHASE vs. OUT-OF-PHASE Out-of-phase waves: Different location Different time When wave peaks and troughs of the waves occur at a different time and/or location  INTERFERENCE When more than one sound wave travels in a medium and arrives at a location at the same time + = Waves lose their individual characteristics and combine to form a new unique wave.  INTERFERENCE Constructive interference: Interference of in-phase waves + = In-phase waves combine to form a wave that is larger than any of the original waves  INTERFERENCE Destructive interference: Interference of out-of-phase waves + = Out-of-phase waves combine to form a wave that is smaller than any of the original waves  INTERFERENCE Destructive interference: Interference of out-of-phase waves with equal amplitude… + = Complete destruction of the wave may occur  1.Period 2.Frequency 3.Amplitude 4.Power ACOUSTIC 5.Intensity PARAMETERS 6.Wavelength 7.Propagation Speed  ACOUSTIC PARAMETERS Acoustic parameters: Describe the features of a sound wave Some parameters are determined by the sound source (i.e., ultrasound machine, transducer), the medium (i.e., tissue), or both.  1. PERIOD Period: The time from the start of one cycle to the start of the next cycle Period  1. PERIOD Units Typical Values Units of time.06-.5 µs Determined by… Adjustable? Sound source No  2. FREQUENCY Frequency: Number of events that occur in a specific duration of time; cycles per second 1 sec 1 sec 6 cycles/sec 3 cycles/sec  2. FREQUENCY Units Typical Values Hertz (Hz) or 1 2 MHz-15 MHz cycle/sec Determined by… Adjustable? Sound source No  PERIOD AND FREQUENCY Period and frequency are reciprocally related Period Frequency ↑ ↓  PERIOD AND FREQUENCY Period x Frequency = 1 When two reciprocal parameters are multiplied together, the result is 1 Period 1/6 sec 1 sec Frequency 6 Hz 1/6 sec x 6 Hz = 1  LET’S PRACTICE… 1 ms Determine: period, frequency, and reciprocal equation with units  1 ms PERIOD ½ ms FREQUENCY 2 kHz RECIPROCAL ½ ms x 2 kHz = 1 EQUATION  LET’S PRACTICE… 1 µs Determine: period, frequency, and reciprocal equation with units  1 µs PERIOD ½ µs FREQUENCY 2 MHz RECIPROCAL ½ µ x 2 MHz = 1 EQUATION  ”BIGNESS PARAMETERS” There are three acoustic parameters that describe the amplitude, strength, or “bigness” of an ultrasound wave: Amplitude Power Frequency  3. AMPLITUDE Amplitude: How big a wave is. Difference between the maximum and baseline value of a wave. Amplitude  3. AMPLITUDE Peak-to-Peak Amplitude: Difference between maximum and minimum values of an acoustic variable. Twice the value of amplitude. Amplitude Peak-to-Peak Amplitude  3. AMPLITUDE Units Typical Values Any units of any Pressure acoustic variable amplitude: 1 MPa-3 MPa Determined by… Adjustable? Sound source Yes  4. POWER Power: Rate of energy transfer. Also describes the bigness of the wave.  4. POWER Units Typical Values Watts (W).004-.09 W Determined by… Adjustable? Sound source Yes POWER AND AMPLITUDE Power and amplitude are directly Power Amplitude ↑ ↑ POWER AND AMPLITUDE Power  Amplitude² Power is proportional to amplitude squared Power 5W Amplitude (Pa) 5 x 5 = 25 Pa  LET’S PRACTICE… Power.5 W Amplitude (Pa) Determine: amplitude Power.5 W Amplitude.25 Pa (Pa).5² =.25.5 x.5 =.25 Amplitude:.25 Pa  LET’S PRACTICE… Power 13 W Amplitude (Pa) Determine: amplitude Power 13 W Amplitude 169 Pa (Pa) 13² = 169 13 x 13 = 169 Amplitude: 169 Pa  5. INTENSITY Intensity: Concentration of energy in a sound beam. Also describes the bigness of the wave. Describes how the power in a wave is distributed in a space.  5. INTENSITY Units Typical Values W/cm².01-300 W/cm² Determined by… Adjustable? Sound source Yes  5. INTENSITY Depends on the power in the beam and the area over which the power is applied Power (W) Intensity (W/cm²) = Area (cm²)  LET’S PRACTICE… Power 2W Area 4 cm² Determine: intensity  Power 2W Area 4 cm² Power (W) 2W Intensity (W/cm²) = =.5 W/cm² Area (cm²) 4 cm² Intensity:.5 W/ cm² POWER AND INTENSITY Intensity  Power Power is proportional to intensity Power 5W Intensity 5 W/cm² AMPLITUDE AND INTENSITY Intensity  Amplitude² Intensity is proportional to amplitude squared Intensity 5 W/cm² Amplitude 25 Pa  BIGNESS PARAMETERS RELATIONSHIPS When defining the relationship between the three bigness parameters: Amplitude Power Intensity Remember…the square (²) always follows amplitude  6. WAVELENGTH Wavelength: Length of one complete cycle Wavelength  6. WAVELENGTH Units Typical Values mm, meters, etc..1-.8 mm Determined by… Adjustable? Both sound source ter No arame and medium determianned medium O n ly p d by both source  WAVELENGTH AND FREQUENCY Wavelength and frequency are inversely related Wavelength Frequency ↑ ↓ WAVELENGTH AND FREQUENCY Wavelength and frequency as they relate to penetration Waveleng Frequenc Increased th y Penetrati on ↑ ↓ ↑ WAVELENGTH AND FREQUENCY Sound with a frequency of 1 MHz has a wavelength of 1.54 mm in soft tissue  WAVELENGTH AND FREQUENCY Equating the relationship between wavelength and frequency in soft tissue… 1.54 mm/µs Wavelength (mm) = Frequency (MHz)  LET’S PRACTICE… Speed of sound in 1.54 mm/µs soft tissue Frequency Curved GE transducer (highest Hz) Determine: wavelength  Speed of sound in 1.54 mm/µs soft tissue Frequency 5 Hz 1.54 mm/µs 1.54 mm/µs Wavelength (mm) = =.308 mm Frequency (MHz) 5 MHz Wavelength:.308 mm  LET’S PRACTICE… Speed of sound in 1.54 mm/µs soft tissue Frequency TV GE transducer (highest Hz) Determine: wavelength Speed of sound in 1.54 mm/µs soft tissue Frequency 9 Hz 1.54 mm/µs 1.54 mm/µs Wavelength (mm) = =.171 mm Frequency (MHz) 9 MHz Wavelength:.171 mm  LET’S PRACTICE… GE Curved Transducer GE TV Transducer Wavelength Wavelength.308 mm.171 mm Draw what the differences in these waves might look like. What does the wavelength tell you about how the sound travels into the body? GE Curved Transducer GE TV Transducer Wavelength Wavelength.308 mm.171 mm Longer wavelengths Shorter wavelengths ↑ Penetration ↓ Penetration  7. PROPAGATION SPEED Propagation Speed: Rate at which a sound wave travels through a medium. Cystic fluid Bone  7. PROPAGATION SPEED Units Typical Values m/s, mm/µs, etc. 500-4000 m/s Determined by… Adjustable? Medium No SPEED OF SOUND IN SOFT TISSUE 1,540  LET’S PRACTICE… Speed of sound in mm/µs soft tissue Speed of sound in km/s soft tissue Determine: speed of sound in soft tissue determined by the units above Speed of sound in 1.54 mm/µs soft tissue Speed of sound in 1.54 km/s soft tissue  7. PROPAGATION SPEED PROPAGATION SPEEDS Air Liquids Soft Tissue Solids Slowest Fastest i.e., bowel gas i.e., urine i.e., organ tissue i.e., bone  7. PROPAGATION SPEED Speed (m/s) = Frequency (Hz) x Wavelength (m) LET’S PRACTICE… Determine the speed of the GE curved and TV probe (we assume the sound is traveling through soft tissue at 1,540 m/s) *Pay attention to units*  LET’S PRACTICE… Transducer Frequency Wavelength GE Curved 5 MHz.308 mm Probe (5,000,000 Hz) (.000308 m) Speed (m/s) = Frequency (Hz) x Wavelength (m) 1,540 m/s = 5,000,000 Hz x.000308 mm GE TV Probe 9 MHz.171 mm (9,000,000 Hz) (.000171 m) Speed (m/s) = Frequency (Hz) x Wavelength STIFFNESS AND DENSITY  STIFFNESS AND DENSITY Stiffness and Density: Both are characteristics of a medium that determine the speed of sound Stiffness Density Ability of an object to resist Relative weight of a material. compression. e h Has t t a t e s gre e on e n c influ ed Which one is stiffer, the marshmall ow or the stick? Which one is denser, the marshmall ow or the stick?  STIFFNESS AND DENSITY Stiffness and density affect on speed: Stiffness Density Stiffness and speed are Density and speed are directly related inversely related Stiffness Speed Density Speed ↑ ↑ ↑ ↓  LET’S PRACTICE… Determine which type of materials have the fastest speed (related to stiffness and density) Determine which type of materials have the slowest speed (related to stiffness and density)  LET’S PRACTICE… Fastest Stiff, not speed dense Slowest Not stiff, speed dense PULSED SOUND  PULSED SOUND Pulsed sound: A pulse of ultrasound is a collection of cycles that travel together.  PULSED SOUND Pulsed sound has two components: 1. Transmit, talking, or “on” time 2. Receive, listening, or “off” time OFF ON ON  1. Pulse Duration 2. Pulse Repetition Period 3. Pulse Repetition PULSED WAVE Frequency PARAMETERS 4. Duty Factor 5. Spatial Pulse Length  1. PULSE DURATION Pulse Duration: Time from the start of a single pulse to the end of that pulse. Pulse Pulse Duration Duration  1. PULSE DURATION Units Typical Values Time (i.e., µs).3-2.0 µs Determined by… Adjustable? Sound source No  1. PULSE DURATION Pulse Duration (µs) = # cycles x period (µs) LET’S PRACTICE… Determine the pulse duration 3 µs  3 µs # cycles 2 Period 3 µs Pulse Duration (µs) = # cycles x period (µs) 6 µs = 2 x 3 µs Pulse Duration = 6 µs 1. PULSE DURATION Pulse # Cycles in Period Frequency Duration the Pulse ↑ ↑ ↑ ↓ Pulsed # cycles Duration (µs) = Frequency (MHz)  LET’S PRACTICE… Determine the pulse durations of both the GE curved and TV transducers in conjunction with the diagram below: GE Curved Transducer Wavelength GE TV Transducer Wavelength 5 MHz 9 MHz 2 cycles/ pulse Curved TV.4 µs = 2 cycles/5 MHz.22 µs= 2 cycles/9 MHz Pulse Duration:.4 µs Pulse Duration:.22 µs  LONG vs. SHORT PULSE DURATIONS Long Pulse Duration Either: Many cycles in the pulse Individual cycles with long periods  LONG vs. SHORT PULSE DURATIONS Short Pulse Duration Either: Few cycles in the pulse Individual cycles with short periods  2. SPATIAL PULSE LENGTH Spatial Pulse Length: Distance from the start of a single pulse to the end of that pulse. Spatial Pulse Spatial Pulse Length Length  2. SPATIAL PULSE LENGTH Units Typical Values Distance (i.e., mm).1-1.0 mm Determined by… Adjustable? Both sound source No and medium 2. SPATIAL PULSE LENGTH Spatial # Cycles in Wavelength Frequency Pulse the Pulse Length ↑ ↑ ↑ ↓ SPATIAL PULSE LENGTH vs. PULSE DURATION Pulse Duration Spatial Pulse Length Time of the Distance of pulse from start pulse from start to finish to finish  LET’S PRACTICE… Determine the two characteristics that create long pulses as they relate to SPL Determine the two characteristics that create short pulses as they relate to SPL Long SPL Many cycles in the pulse Cycles with longer wavelengths Short SPL Fewer cycles in the pulse Cycles with shorter wavelengths  3. PULSE REPETITION PERIOD Pulse Repetition Period: Time from the start of one pulse to the start of the next pulse. 1 “on” time + 1 “off” time = Pulse Repetition Period Pulse Repetition  3. PULSE REPETITION PERIOD Units Typical Values Time (i.e., ms) 100 µs-1 ms Determined by… Adjustable? Sound source Yes Chan g imag ing the e de chan pt ges t h pulse he repe tition pe 3. PULSE REPETITION PERIOD Changing the image depth changes the pulse repetition period Shallow Deep Time from one Time from one pulse to the pulse to the next is short next is long PRP decreased PRP increased 3. PULSE REPETITION PERIOD What can the sonographer adjust? On-Time Off-Time Pulse Repetition Period What is this duration of the wave called? Can this be changed by the sonographer? 3. PULSE REPETITION PERIOD On-Time Off-Time The sonographer can adjust the “off” time or listening time by adjusting the depth. Deep imaging=longer PRP Shallow imaging=shorter Pulse Repetition Period PRP  4. PULSE REPETITION FREQUENCY Pulse Repetition Frequency: Number of pulses that an US system transmits into the body each second. 1 sec  4. PULSE REPETITION FREQUENCY Units Typical Values Hz (pulses/sec) 1,000-10,000 Hz Determined by… Adjustable? Sound source Yes Chan g imag ing the e de chan pt ges t h pulse he repe tition fre 4. PULSE REPETITION FREQUENCY Changing the image depth changes the pulse repetition frequency Shallow Deep PRF high PRF low Inversely related: Inversely related: PRF ↑, depth ↓ PRF ↓, depth ↑  PRP and PRF PRP and PRF are inversely related to one another: Longer Shorter PRP PRP Lower PRF Higher PRF x PRP = 1 PRF When two reciprocal parameters are multiplied together, the result is  5. DUTY FACTOR Duty Factor: Percentage or fraction of time the system PRP transmits a pulse. Pulse Duration Duty Factor = Pulse Duration / PRP  5. DUTY FACTOR Units Typical Values Unitless (%).2 %-.5% Determined by… Adjustable? Sound source Yes Chan ima ging t he ge chan depth g duty es the facto r 5. DUTY FACTOR PULSED WAVE Duty Factor Pulsed Wave =.002-.005 Transmittin ~.2% of the time g Receiving ~ 99.8% of the CONTINUOUS WAVE time Duty Factor Continuous Wave = 1.0 Transmittin 100% of the time g  5. DUTY FACTOR Maximum Duty Factor Minimum Duty Factor 100% 0% CW ultrasound Transducer is off Pulse Duration (ms) Duty Factor (%) = x 100 Pulse Repetition Period (ms)  SHALLOW vs. DEEP IMAGING Shallow Deep Imaging Imaging Less listening More listening Shorter PRP Longer PRP Higher PRF Lower PRF Any Questions??